Metal halide and high-pressure sodium lamps are two examples of lamps which use a discharge vessel made of a ceramic material. The robustness of the ceramic material permits the use of more corrosive chemical fills and allows the discharge vessels to be operated at higher temperatures. A sealing composition, commonly referred to as a frit, is used to form a hermetic seal between the feedthrough sections of the electrodes and the ceramic body of the discharge vessel. The sealing composition may also be used to join together the ceramic pieces which form the discharge vessel.
FIG. 1 shows a discharge vessel 1 for a conventional ceramic metal halide lamp. The discharge vessel 1 includes a hollow ceramic body 6 filled with a chemical fill 8 and into which electrodes 14 are fed through ceramic capillaries 2. The discharge vessel has halves 17a,b that are joined at seam 5, where the electrode ends 3 are inside the respective halves and extend into the discharge chamber 12. The distal ends of the capillaries are each sealed with a respective frit seal 9. A conventional Al2O3—Dy2O3—SiO2 sealing composition for this purpose is described in U.S. Pat. No. 4,076,991.
Translucent polycrystalline alumina (PCA) has been by far the ceramic material of choice for making ceramic discharge vessels. Yet despite its pervasive use, PCA is not used in some HID applications because it is only translucent and not transparent. In particular, a PCA discharge vessel is generally not suitable for focused-beam, short-arc lamps such as projection lamps and automotive headlights. For these applications, the transparent sapphire (single-crystal) form of aluminum oxide is used. In addition, PCA discharge vessels although superior to quartz arc tubes in metal halide lamps can react with the rare earth halide fills limiting the durability and life of such lamps.
Aluminum oxynitride (AlON) is a transparent ceramic material that has been identified as a potential replacement for PCA. See, for example, Japanese Patent No. 09-92206 and U.S. Pat. Nos. 5,924,904 and 5,231,062. AlON has a cubic spinel structure and a composition that may be generally represented by the empirical formula Al(64+x)/3O32−xNx where 2.75≦x≦5. The mechanical strength and thermal expansion of AlON are close to those of PCA, so that AlON should be able to survive the stresses in high-intensity discharge lamps.
Aluminum nitride (AlN) has been shown to be more resistant to the corrosive effects of rare earth metal halide fills than polycrystalline aluminum oxide. Although the fully-dense sintered AlN ceramic is only translucent and not transparent, the superior corrosion resistance is desirable for metal halide lamps. For example, the use of AlN arc discharge vessels for ceramic metal halide lamps is described in European Patent Application No. 0371315A1 and PCT Application No. WO 03/060952.
Despite the potential advantages of aluminum nitride and aluminum oxynitride ceramics, there remain a number of technical difficulties which must be overcome for these materials to be considered for use in HID lamp applications. One in particular is the reaction of these ceramics with the conventional silica-containing glass/ceramic frit materials used to seal the discharge vessels. As described above, the function of the frit is to hermetically seal the ceramic body of the discharge vessel, in particular to the feedthrough portion of the electrode assembly. During the sealing operation, contact between the molten frit and the aluminum nitride or aluminum oxynitride ceramic induces a reaction that releases nitrogen gas. Since most of the nitrogen evolved from the reaction cannot be accommodated in the molten frit, it escapes as gas bubbles into the frit seal. These gas bubbles may degrade the quality and function of the hermetic seal leading to premature failure of the lamp, particularly when higher pressures are present in the discharge vessel.
U.S. Pat. No. 7,362,053 describes a method for minimizing bubble formation in the seal region of an aluminum oxynitride discharge vessel by forming a less reactive surface layer in the seal region. In one embodiment, this is accomplished by heating at least the seal region of the discharge vessel in a reducing atmosphere. In another embodiment, an aluminum oxide layer is deposited in the seal region to act as a barrier between the molten frit and the aluminum oxynitride vessel during sealing.
While such methods are effective, it would be simpler and more advantageous if a sealing frit composition could be formulated in such a way to reduce or eliminate bubble formation in the seal without the need for additional treatments on the sintered discharge vessel.